This invention relates to a disk cartridge encasing such a disk as a magnetic disk, an optical disk, etc, and more particularly to an improvement of materials for a shutter to be provided on the outside of a cartridge case to open or close head insert openings of the cartridge case.
The shutters so far used are made from a thin stainless steel sheet and have a U-shaped cross-section. The shutters have an opening substantially the same shape as that of the head insert openings, engaging lugs for preventing the shutter from slipping out of the case, a spring-fixing lug, etc. on the shutter side plates. The metallic shutters have no problem with respect to their strength, but are liable to scrape the plastic cartridge case surface through contact with the peripheral edge, lugs, etc. of the shutters, and the scrapings are liable to fall into the cartridge case and deposit onto the disk or head, causing a dropout, etc.
To overcome such inconveniences as above and also to reduce the production cost, shutters molded from plastics have been proposed [for example, Japanese Patent Application Kokai (laid-open) No. 60-231985], where the upper side plate and the lower side plate, which are juxtaposed to each other in the case thickness direction, and an end plate connected to the upper and lower side plates at one end are integrally molded by injection molding. Generally, the reasons for using a thin stainless steel sheet (thickness: 0.2-0.3 mm) as a material for forming a shutter in the prior art are that the thin stainless steel sheet can satisfy requirements for strength, such as mechanical strength, wearing resistance, etc. and requirements for smaller sizes and smaller thickness at the same time, and further that it undergoes no rusting and thus can give an aesthetic merit. Needless to say, it is desirable that a shutter made from plastics must also satisfy the similar structural and functional requirements.
With recent developments of novel molding plastic materials and their compounding with other materials, molded plastic articles have been given higher functions. Furthermore, with improvements of molding technique, etc., even plastic articles of much complicated shape can be molded with high precision. That is, even those articles that have been so far regarded as unmoldable can be now molded from plastics. However, it has been still difficult to replace the shutter made from a thin stainless steel sheet with a plastic-molded shutter, since the requirements for the production technique or for the strength have not been fully satisfied.
The most important problem in the molding technique resides in the extremely thin upper and lower side plates constituting a shutter, and the large area of the side plates, as compared with the plate thickness. In other words, when molten resin is to be poured into a thin molding space of much large surface area, the molten resin cannot be uniformly and surely extended throughout the molding space, for example, even by elevating the pouring pressure to a considerable degree, since the surface area is too large. Furthermore, since the distance between the upper and lower side plates is as small as 2-3 mm, the core placed between the upper and lower side plates to partition one off another cannot be cooled. That is, there is another problem that mold cooling cannot be carried out uniformly.
The shutter for use in the disk cartridge has a U-shaped cross-section, where the upper and lower side plates are supported substantially at their joint parts to the end plate like cantilevers, and thus large moments are liable to develop at the joint parts. That is, there is a structural problem such as easy breakage. Still furthermore, the shutter must thoroughly withstand even an impact load at falling and also must have a good sliding characteristic.